Artificial salt-water solar ponds are presently used as solar collectors in order to provide a source of low-grade heat for conversion into electricity. Such ponds have a three-layer regime: an upper, convective, wind-mixed layer at the surface, with a uniform salinity of 3-5%, and with a depth of 30-50 cm., depending upon wind conditions; and intermediate, non-convective layer, termed a halocline, about 1-1.5 m. deep, with a salinity that increases uniformly with depth from about 5% near the top to about 30% near the bottom; and a lower heat-storage layer, from 3-5 m. deep, depending on the amount of heat storage desired with a uniform salinity of about 30%. Solar radiation incident on the surface is absorbed within the layers. Heat absorbed within a stratum of the wind-mixed layer reduces the density of the stratum, and creates buoyant water than quickly reaches the surface, dissipating the absorbed heat into the atmosphere. Thus, the temperature of the wind-mixed layer approximates ambient temperature. However, heat absorbed in the halocline and in the heat-storage layer is trapped in these layers. The halocline is non-convective, because the density of a stratum is so large, due to its salinity, that any decrease in density due to its temperature rise as a consequence of the absorption of solar radiation is insufficient to materially change its buoyancy, with the result that solar radiation establishes a temperature profile in the solar pond that matches its salinity profile. The halocline thus serves as a transparent, insulating cover for the heatstorage layer, and protects the latter against conductive heat loss to the atmosphere.
From actual experience with solar ponds, the halocline is remarkably stable over long periods of time, because the rate of salt diffusion is so slow. However, the halocline is particularly sensitive to the effects of wind at the surface. Gusts of wind that create surface waves increase mixing at the surface, and increase the depth of the wind-mixed layer at the expense of the halocline. This has the effect of reducing the rate at which heat can be added to the heat-storage layer from solar radiation, because the deeper the wind-mixed layer, the greater is the percentage of heat absorbed within this layer, all of this heat being lost to the atmosphere. In order to reduce this problem, windbreaks are conventionally placed on the surface of the pond; and, to this end, nets of low-density polypropylene, for example, have been floated on the surface. These nets are anchored on the banks or rim of the pond, and have proven to be effective in reducing the effects of wind-mixing. However, a number of problems have developed. First of all, the cross-pieces of the nets must be relatively close to suppress wave formation, with the result that the shadow cast by the nets may represent as much as 5% of the total area of the pond; and the nets thus reduce the efficiency of the pond as a collector. In addition, salt precipitates collect on the exposed surfaces of the nets, increasing their weight, and eventually causing them to sink below the surface, thereby reducing the effectiveness of the nets to suppress waves. Thus, the shadows cast by conventional windbreaks, and the maintenance required as a consequence of precipitates collecting thereon, constitute problems that heretofore have resisted solution.
It is, therefore, an object of the present invention to provide a new and improved method of and apparatus for protecting a halocline in a salt-water solar pond against the effectiveness of wind, wherein the deficiencies of the prior art are overcome or substantially reduced.